The purposes of this study are to determine the maximum tolerated dose of L-buthionine sulfoximine (BSO) given as a bolus and continuous infusion in conjunction with a bolus of melphalan (L-PAM), determine pharmacokinetics and pharmacodynamics of BSO, to explore biochemical effects of BSO and L-PAM on GSH metabolism, and to assess toxicities of BSO and L-PAM.